Why male dark fishing spiders spontaneously die after sex

It's an unusually cruel fate even in the twisted world of spider
sex: when male dark fishing spiders copulate, they die --
death is the unavoidable consequence of the mechanism by which
males transfer sperm to females.

Dark fishing spiders are the first non-web-weaving spider in which obligatory death after
mating has been observed, reports a team of scientists on
19 June in Biology Letters. Only three other spider
species share this characteristic. The inescapable outcome
makes these spiders a type of rare monogynous species -- one in
which males mate only once, ever -- and means that maximising the
chance of reproductive success becomes a crucial component of the
male's behaviour.

"The act of sperm transfer is triggering this cascade of death,"
said Steven
Schwartz, a behavioural ecologist at the University of Nebraska
and coauthor of the study. "Once that button is pushed, it's lights
out." It's relatively common in spider species for females to
kill and eat males after mating, but Schwartz has yet to catch a
female dark fishing spider making a kill herself.

Schwartz, now a postdoc, began graduate school hoping to find
and study a local spider species with some interesting
behaviours. In 2006 he began collecting dark fishing spiders
(Dolomedes tenebrosus) from creeks near the university.
Normally, these arachnids hang out near water, where they wait for
telltale aquatic vibrations produced by prey such as insects,
tadpoles, and small fish. Then, like Jesus lizards, they chase their prey across the water's
surface, occasionally diving below when threatened.

Female fishing spiders are about 3 inches (7.6 centimetres)
across and 14 times heavier than the relatively puny males, which
might span an inch (2.5 centimetres) from tip to tip. Finding
the arachnids is easiest at night, when their eight eyes shine like
tiny holidays lights in the dark, sometimes revealing as many
as 50 individuals in
an area the size of a parking space.

Obtaining permission to wade around in people's backyards
overnight took some work. "I just want to be in your creek at
night, all night long, watching these spiders and what they do,"
Schwartz would tell landowners; they agreed, and the local park
police quickly learned that the dude in chest waders spending
evenings near the creeks was looking for spiders, not trouble.

What Schwartz saw, again and again, was that after a bit of
courtship and a quick thrust of the pedipalp, the males died.
Every time. "I have not found a male yet that has not died when he
copulates," Schwartz said, estimating that he's observed nearly 300
such matings.

The surprise came when Schwartz realised it wasn't the females
who were killing the males, as has been observed in other
species. Rather, as soon as a male began transferring sperm,
he curled up and became unresponsive, hanging from the female's
genital opening until she removed or ate him.

The reason is an unusual quirk of the male dark fishing spider's
physiology. During mating, a male inflates what's called a
hematodochal bulb inside the pedipalp, the appendage he uses to
deposit sperm in the female. In most species, the bulb can be
deflated after mating. In dark fishing spiders, it's irreversibly
inflated, resulting in a sudden shift in blood pressure that causes
the male to curl up and leaves him immobilised and stuck to the
female - doomed but not immediately dead.

When Schwartz separated the mated males from their giant
partners, he found that the circulatory system of the males
continued to pump blood for an average of almost three hours.
"Technically, they're not dead yet, but they're on that trajectory
and they're not coming back," Schwartz said. Sex, for these
spiders, "is a one-shot deal."

Female dark fishing spiders always cannibalise their mates,
mostly commencing their meals within 20 minutes of mating. Schwartz
says his next set of publications will describe the cannibalisation
and potential reproductive benefits for the male. "We do have
evidence that by dying, and then through sexual
cannibalisation, [the male] increases the number and size of his
own offspring," Schwartz said.

It's a strange arrangement, but one that's evolved multiple
times, both within and outside of the web-weaving family. "The
study provides another independent origin of a monogynous mating
system," said Jutta Schneider, a behavioural ecologist at the University of
Hamburg, who has modelled the complicated trade-offs needed for
monogynous systems to evolve. "I would expect that males evolved
measures of paternity protection and I am curious to see what the
mechanism will be D. tenebrosus." Some of these
mechanisms include such charming measures as mate-plugging by
genital mutilation, where males will break off pedipalps and leave
them in the female.

This story originally appeared on Wired.com.
Click through for a video of the dark fishing spider